This invention relates to a noise suppressor and a noise suppression method. It relates particularly to a mobile terminal incorporating a noise suppressor for suppressing noise in a speech signal. A noise suppressor according to the invention can be used for suppressing acoustic background noise, particularly in a mobile terminal operating in a cellular network.
One purpose of noise suppression or speech enhancement in a mobile telephone terminal is to reduce the impact of environmental noise on a speech signal and thus to improve the quality of communication. In the case of an up-link (transmission, TX) signal, it is also desired to minimise detrimental effects in the speech coding process caused by this noise.
In face-to-face communication, acoustic background noise disturbs a listener and makes it more difficult to understand speech. Intelligibility is improved by a speaker raising his or her voice so that it is louder than the background noise. In the case of telephony, background noise is troublesome because there is no additional information provided by facial expressions and gestures.
In digital telephony, a speech signal is first converted into a sequence of digital samples in an analogue-to-digital (A/D) converter and then compressed for transmission using a speech codec. The term codec is used to describe a speech encoder/decoder pair. In this description, the term xe2x80x9cspeech encoderxe2x80x9d is used to denote the encoding side of the speech codec and the term xe2x80x9cspeech decoderxe2x80x9d is used to denote the decoding functions of the speech codec. It should be appreciated that a general speech codec may be implemented as a single functional unit, or as separate elements that implement the encoding and decoding operations.
In digital telephony, the deleterious effect of background noise can be great. This is due to the fact that speech codecs are generally optimised for efficient compression and acceptable reconstruction of speech and their performance can be impaired if noise is present in the speech signal, or errors occur in speech transmission or reception. In addition, the presence of noise itself can lead to distortion to the background noise signal when it is encoded and transmitted.
Impaired performance of a speech codec reduces both the intelligibility of the transmitted speech and its subjective quality. Distortion of the transmitted background noise signal degrades the quality of the transmitted signal, making it more annoying to listen to and rendering contextual information less recognisable by changing the nature of the background noise signal. Consequently, work in the field of speech enhancement has concentrated on studying the effect of noise on speech coding performance and producing pre-processing methods to reduce the impact of noise on speech codecs.
The problems discussed above relate to arrangements in which only one microphone is present to provide only one signal. In such arrangements a noise suppressor is provided which can interpret the one-channel signal to decide which parts of it represent underlying speech and which represent noise.
When a digital mobile terminal receives an encoded speech signal, it is decoded by the decoding part of the terminal""s speech codec and supplied to a loudspeaker or earpiece for the user of the terminal to hear. A noise suppressor may be provided in the speech decoding path, after the speech decoder, in order to reduce the noise component in the received and decoded speech signal. However, in noisy conditions the performance of the speech decoder may be affected detrimentally, resulting in one or more of the following effects:
1. The speech component of the signal may sound less natural or harsh, as critical information required by the speech codec in order to correctly decode the speech signal is altered by the presence of noise.
2. The background noise may sound unnatural because codecs are generally optimised for compressing speech rather than noise. Typically this gives rise to increased periodicity in the background noise component and may be sufficiently severe to cause the loss of contextual information carried by the background noise signal.
Information about an encoded speech signal may also be lost or corrupted during transmission and reception, for example due to transmission channel errors. This situation may give rise to further deterioration in the speech decoder output, causing additional artefacts to become apparent in the decoded speech signal. When a noise suppressor is used in the speech decoding path, after a speech decoder, non-optimal performance of the speech decoder may in turn cause the noise suppressor to operate in a less than optimal manner.
Therefore special care must be taken when implementing noise suppressors intended to operate on decoded speech signals. In particular, two conflicting factors have to be balanced. If the noise suppressor provides too much noise attenuation, this may reveal the deterioration in speech quality caused by the speech codec. However, due to the intrinsic properties of typical speech codecs, which are optimised for the encoding and decoding of speech, decoded background noise can sound more annoying than the original noise signal and so it should be attenuated as much as possible. Thus, in practice, it is found that a slightly lower level of noise reduction may be optimal for decoded speech signals, compared with that which can be applied to speech signals prior to encoding.
It is generally desirable that when noise suppression is used during speech encoding and/or decoding, it should reduce the level of background noise, minimise the speech distortion caused by the noise reduction process and preserve the original nature of the input background noise.
An embodiment of a mobile terminal comprising a noise suppressor according to prior art will now be described with reference to FIG. 1. The mobile terminal and the wireless system with which it communicates operate according to the Global System for Mobile telecommunications (GSM) standard. FIG. 1 shows a mobile terminal 10 comprises a transmitting (speech encoding) branch 12 and a receiving (speech decoding) branch 14.
In the transmitting (speech encoding) branch, a speech signal is picked up by a microphone 16 and sampled by an analogue-to-digital (A/D) converter 18 and noise suppressed in a noise suppressor 20 to produce an enhanced signal. This requires the spectrum of the background noise to be estimated so that background noise in the sampled signal can be suppressed. A typical noise suppressor operates in the frequency domain. The time domain signal is first transformed to the frequency domain, which can be carried out efficiently using a Fast Fourier Transform (FFT). In the frequency domain, voice activity has to be distinguished from background noise, and when there is no voice activity, the spectrum of the background noise is estimated. Noise suppression gain coefficients are then calculated on the basis of the current input signal spectrum and the background noise estimate. Finally, the signal is transformed back to the time domain using an inverse FFT (IFFT).
The enhanced (noise suppressed) signal is encoded by a speech encoder 22 to extract a set of speech parameters which are and then channel encoded in a channel encoder 24 where redundancy is added to the encoded speech signal in order to provide some degree of error protection. The resultant signal is then up-converted into a radio frequency (RF) signal and transmitted by a transmitting/receiving unit 26. The transmitting/receiving unit 26 comprises a duplex filter (not shown) connected to an antenna to enable both transmission and reception to occur.
A noise suppressor suitable for use in the mobile terminal of FIG. 1 is described in published document WO97/22116.
In order to lengthen battery life, different kinds of input signal-dependent low power operation modes are typically applied in mobile telecommunication systems. These arrangements are commonly referred to as discontinuous transmission (DTX). The basic idea in DTX is to discontinue the speech encoding/decoding process in non-speech periods. DTX is also intended to limit the amount of data that is transmitted over the radio link during pauses in speech. Both measures tend to reduce the amount of power consumed by the transmitting device. Typically, some kind of comfort noise signal, intended to resemble the background noise at the transmitting end, is produced as a replacement for actual background noise. DTX handlers are well known in the art such as the GSM Enhanced Full Rate (EFR), Full Rate and Half Rate speech codecs.
Referring again to FIG. 1, the speech encoder 22 is connected to a transmission (TX) DTX handler 28. The TX DTX handler 28 receives an input from a voice activity detector (VAD) 30 which indicates whether there is a voice component in the noise suppressed signal provided as the output of the noise suppressor block 20. The VAD 30 is basically an energy detector. It receives a filtered signal, compares the energy of the filtered signal with a threshold and indicates speech whenever the threshold is exceeded. Therefore, it indicates whether each frame produced by the speech encoder 22 contains noise with speech present or noise without speech present. The most significant difficulty in detecting speech in a signal generated by a mobile terminal is that the environments in which such terminals are used often lead to low speech/noise ratios. The accuracy of the VAD 30 is improved by using filtering to increase the speech/noise ratio before the decision is made as to whether speech is present.
Of all the environments in which mobile telephones are used, the worst speech/noise ratios are generally encountered in moving vehicles. However, if the noise is relatively stationary for extended periods, that is, if the noise amplitude spectrum does not vary much in time, it is possible to use an adaptive filter with suitable coefficients to remove much of the vehicle noise.
The noise levels in environments where mobile terminals are used may change constantly. The frequency content (spectrum) of the noise may also change, and can vary considerably depending on circumstances. Because of these changes, the threshold and adaptive filter coefficients of the VAD 30 must be constantly adjusted. To provide reliable detection, the threshold must be sufficiently above the noise level to avoid noise being falsely identified as speech, but not so far above it that low level parts of speech are identified as noise. The threshold and the adaptive filter coefficients are only up-dated when speech is not present. Of course, it is not prudent for the VAD 30 to up-date these values on the basis of its own decision about the presence of speech. Therefore, this adaptation only occurs when the signal is substantially stationary in the frequency domain, but does not have the pitch component inherent in voiced speech. A tone detector is also used to prevent adaptation during information tones.
A further mechanism is used to ensure that low level noise (which is often not stationary over long periods) is not detected as speech. In this case, an additional fixed threshold is used so that input frames having frame power below the threshold are interpreted as noise frames.
A VAD hangover period is used to eliminate mid-burst clipping of low level speech. Hangover is only added to speech-bursts which exceed a certain duration to avoid extending noise spikes. Operation of a voice activity detector in this regard is known in the art.
The output of the VAD 30 is typically a binary flag which is used in the TX DTX handler 28. If speech is detected in a signal, its transmission continues. If speech is not detected, transmission of the noise suppressed signal is stopped until speech is detected again.
In most mobile telecommunication systems, DTX is mostly applied in the up-link connection since speech encoding and transmission is typically much more power consuming than reception and speech decoding, and because the mobile terminal typically relies on the limited energy stored in its battery. During periods in which there is no transmission of a signal supposedly carrying speech, comfort noise is generated to give the listener an illusion that the signal is, in fact, continuous. As described in further detail below, in some cellular telephone systems, comfort noise is generated in the receiving terminal, on the basis of information received from the transmitting terminal describing the characteristics of the noise at the transmitting terminal.
Generally, an explicit flag is provided in the speech decoder indicating whether the DTX operation mode is on or not. This is the case with, for example, all of the GSM speech codecs. Other cases exist, however, for example Personal Digital Cellular (PDC) networks, where a frame repeating mode must be activated in the noise suppressor by comparing input frames to earlier ones and setting up a voice operated switch (VOX) flag if consecutive frames are identical. Furthermore, in a mobile-to-mobile connection, no information is provided in the down-link connection about the occurrence of DTX in the up-link connection.
In some speech codecs, such as the GSM EFR codec, the decision to switch off transmission during pauses in speech is made in a DTX handler of the speech encoder. At the end of a speech burst, the DTX handler uses a few consecutive frames to generate a silence descriptor (SID) frame which is used to carry comfort noise parameters describing estimated background noise characteristics to the decoder. A silence descriptor (SID) frame is characterised by an SID code word.
After transmission of an SID frame, radio transmission is cut and a speech flag (SP flag) is set to zero. Otherwise, the SP flag is set to 1 to indicate radio transmission. The SID frame is received by the speech decoder, which then generates noise with a spectral profile corresponding to the properties described in the SID frame. Occasional SID frame updates are transmitted to the decoder to maintain a correspondence between the background noise at the transmitting terminal and the comfort noise generated in the receiving terminal. For example, in a GSM system, a new SID frame is sent once every 24 frames of normal transmission. Providing occasional SID frame updates in this way not only enables the generation of acceptably accurate comfort noise, but also significantly reduces the amount of information that must be transmitted over the radio link. This reduces the bandwidth required for transmission and aids efficient use of radio resources.
In the receiving (speech decoding) branch 14 of the mobile terminal, an RF signal is received by the transmitting/receiving unit 26 and down-converted from RF to base-band signal. The base-band signal is channel decoded by a channel decoder 32. If the channel decoder detects speech in the channel decoded signal, the signal is speech decoded by a speech decoder 34.
The mobile terminal also comprises a bad frame handling unit 38 to handle bad (i.e. corrupted) frames. A bad traffic frame is flagged by the Radio Sub-System (RSS) by setting a Bad Frame Indication (BFI) to 1. If errors occur in the transmission channel, normal decoding of lost or erroneous speech frames would give rise to a listener hearing unpleasant noises. To deal with this problem, the subjective quality of lost speech frames is typically improved by substituting bad frames with either a repetition or an extrapolation of a previous good speech frame or frames. This substitution provides continuity of the speech signal and is accompanied by a gradual attenuation of the output level, resulting in silencing of the output within a rather short period. A good traffic frame is flagged by the radio subsystem with a BFI of 0.
An embodiment of a prior art bad frame handling unit 38 is located in the Receive (RX) Discontinuous Transmission (DTX) handler. The bad frame handling unit carries out frame substitution and muting when the radio sub-system indicates that one or more speech or Silence Descriptor (SID) frames have been lost. For example, if SID frames are lost, the bad frame handling unit notifies the speech decoder of this fact and the speech decoder typically replaces a bad SID frame with the last valid one. This frame is repeated and gradually attenuated just as in the case of a repeated speech frame, in order to provide continuity to the noise component of the signal. Alternatively, an extrapolation of a previous frame is used rather than a direct repetition.
The purpose of frame substitution is to conceal the effect of lost frames. The purpose of attenuating the output when several frames are lost is to indicate the possible breakdown of the radio link (channel) to the user and to avoid generating possibly annoying sounds, which may result from the frame substitution procedure. However, substitution and attenuation of the usually uninformative background noise in the lost frames affects the perceived quality of the noisy speech or the pure background noise. Even at rather low levels of background noise, rapid attenuation of the background noise in lost frames leads to an impression of a badly decreased fluency of the transmitted signal. This impression becomes stronger if the background noise is louder.
The signal produced by the speech decoder, whether decoded speech, comfort noise or repeated and attenuated frames, is converted from digital to analogue form by a digital-to-analogue converter 40 and then played through a speaker or earpiece 42, for example to a listener.
According to an aspect of the invention there is provided a noise suppressor to suppress noise in a signal containing background noise the noise suppressor comprising an estimator to estimate a background noise spectrum in which an indication from at least one of a discontinuous transmission unit and a channel error detector is used to control estimation of the background noise spectrum.
Preferably the indication is provided by a speech decoder in an up-link path in the network.
Preferably the noise suppressor suppresses noise in a signal provided by the speech decoder.
Preferably the indication arises in a channel decoder and is handled by the speech decoder. Preferably the indication in handled by a bad frame handling unit in the speech decoder.
Preferably the noise suppressor provides its noise suppressed signal to a speech encoder.
Preferably the noise suppressor uses a flag or an indication which indicates that individual frames which are used to transmit the signal over the channel are erroneous.
Preferably up-dating of the estimated background noise spectrum is suspended during periods in which channel errors in the signal are detected by the channel error detector. In this way the parts of the signal containing channel errors or parts of the signal which are being generated to mask or ameliorate the channels errors are not used in the production of the estimate of the noise.
Preferably the noise suppressor comprises a voice activity detector to control estimation of the background noise spectrum. Preferably the estimated background noise spectrum is up-dated when the voice activity detector indicates that there is no speech. Preferably the state of the voice activity detector and/or its memory of previous no speech/speech decisions is/are frozen when the channel error detector detects channel errors.
Preferably a comfort noise is generated by a comfort noise generator during time periods in which the signal is not being transmitted. Preferably up-dating of the estimated background noise spectrum is suspended during periods in which the discontinuous transmission unit is indicating that the signal is not being transmitted. In this way the comfort noise is not used in the production of the estimate of the noise.
The term xe2x80x9ccomfort noisexe2x80x9d means a noise generated to represent background noise without being the background noise actually occurring at the time when it is generated. For example, the comfort noise may be a noise estimated from analysing background noise before the comfort noise is generated, it may be a random or pseudo-random noise or it may be a combination of noise estimated from analysing background noise and random or pseudo-random noise.
In an embodiment of the invention in which the noise suppressor is provided in a mobile terminal, it may be located so that it provides noise suppressed speech to an encoder and receives noise suppressed speech from a decoder. Of course, the encoder and decoder may comprise a codec.
Preferably the noise suppressor is in a wireless path. It may be in a down-link wireless path from a communications network to a communications terminal.
According to another aspect of the invention there is provided a method of noise suppression to suppress noise in a signal containing background noise comprising the steps of:
estimating a background noise spectrum;
using the background noise spectrum to suppress noise in the signal;
receiving an indication to indicate the operation of at least one of a discontinuous transmission unit and a a channel error detector; and
using the indication to control estimation of the background noise spectrum.
According to another aspect of the invention there is provided a mobile terminal comprising a noise suppressor to suppress noise in a signal containing background noise the noise suppressor comprising an estimator to estimate a background noise spectrum in which an indication from at least one of a discontinuous transmission unit and a channel error detector is used to control estimation of the background noise spectrum.
Preferably the mobile terminal comprises the channel error detector. The channel error detector may provide an indication that individual frames which are used to transmit the signal over a channel are erroneous.
Preferably the indication is provided by a speech decoder in a down-link path. Preferably the detector for detecting channel errors is in the speech decoder. Preferably the indication arises in a channel decoder and is handled by the speech decoder. Preferably the indication is handled by a bad frame handling unit in the speech decoder.
Preferably the noise suppressor of the mobile terminal comprises a voice activity detector to control estimation of the background noise spectrum. Preferably the voice activity detector is part of a speech encoder.
Preferably the mobile terminal comprises the discontinuous transmission unit.
According to another aspect of the invention there is provided a mobile terminal comprising a downlink path having a receiver to receive wireless signals and a means to output the signal in a form understandable by a user and a noise suppressor to suppress noise in received signals in which the noise suppressor is provided in the downlink path.
When applied to a communications path in a communications system, the term downlink refers to the path from the network to a mobile terminal. Of course, the signals may be transmitted to a fixed communications terminal, such as a landline telephone, rather than to a mobile terminal.
According to another aspect of the invention there is provided a mobile communications system comprising a mobile communications network and a plurality of mobile communications terminals in which the network has a noise suppressor to suppress noise in a signal containing background noise the noise suppressor comprising an estimator to estimate a background noise spectrum in which an indication from at least one of a discontinuous transmission unit and a channel error detector is used to control estimation of the background noise spectrum.
Preferably the signal is produced by a microphone. It may be produced by a telephone microphone.
Preferably the mobile communications system comprises the discontinuous transmission unit.
Preferably the noise suppressor is located at the output of a decoder in the network so as to suppress noise in decoded speech. Alternatively the noise suppressor provides noise suppressed speech to an encoder in the network.
According to another aspect of the invention there is provided a mobile communications system comprising a mobile communications network and a plurality of mobile communications terminals in which a noise suppressor is provided in the network to suppress noise in signals provided by at least one of the mobile terminals.
According to another aspect of the invention there is provided a frame replacer for replacing frames in a signal to limit the disturbance caused by channel errors in the signal the frame replacer comprising a memory to store a previously received part of the signal indicated as being free of errors a noise generator to generate a noise signal and a frame generator to progressively attenuate the previously received part of the signal and to combine the attenuated previously received part of the signal and the noise signal to produce a combined signal the frame generator providing to the combined signal an increasing contribution from the noise signal relative to the previously received part of the signal as time passes.
The noise signal may be a random or pseudo-random signal. It may be a combination of a random or pseudo-random signal and a noise estimate.
Preferably the previously received part of the signal is repeated and progressively attenuated on each repetition. It may be a frame which has been received. The noise signal may be a set of synthetic frames which have been generated. The synthetic frames of the noise signal may be added frame by frame to each progressively attenuated frame of the previously received part of the signal. Preferably the contribution of the noise signal is increased to the same extent as the previously received part of the signal is reduced so that the level of the combined signal is about the same as the previously received part of the signal.
At least one of the noise signal and previously received part of the signal is attenuated so as to indicate breakdown of the channel. Preferably both signals are attenuated. Attenuation of the noise signal may commence once the previously received part of the signal is attenuated to such an extent that it no longer contributes to the combined signal.
The frame replacer may be part of a bad frame handler which is a part of a speech decoder. The noise generator may be in a noise suppressor. The noise suppressor may obtain information from the speech decoder and may adjust the amplification it applies to the noise it has generated based on the information it receives and its own measurement of how much attenuation the repeated/interpolated frames have undergone since the latest time when the bad frame indication was off.
The replacer may replace frames containing errors, missing frames or both. The channel errors may have been caused by transmission of the signal over an air interface.
According to another aspect of the invention there is provided a method for replacing frames in a signal to limit the disturbance caused by channel errors the method comprising the steps of:
storing a previously received part of the signal indicated as being free of errors;
progressively attenuating the previously received part of the signal;
generating a noise signal;
combining the attenuated previously received part of the signal and the noise signal to produce a combined signal;
providing to the combined signal an increasing contribution from the noise signal relative to the previously received part of the signal as time passes.
According to another aspect of the invention there is provided a mobile terminal comprising a frame replacer for replacing frames in a signal to limit the disturbance caused by the channel errors in the signal the frame replacer comprising a memory to store a previously received part of the signal indicated as being free of errors a noise generator to generate a noise signal and a frame generator to progressively attenuate the previously received part of the signal and to combine the attenuated previously received part of the signal and the noise signal to produce a combined signal the frame generator providing to the combined signal an increasing contribution from the noise signal relative to the previously received part of the signal as time passes.
According to another aspect of the invention there is provided a communications system comprising a communications network having a frame replacer for replacing frames in a signal to limit the disturbance caused by channel errors and a plurality of communications terminals the frame replacer comprising a memory to store a previously received part of the signal indicated as being free of errors a noise generator to generate a noise signal and a frame generator to progressively attenuate the previously received part of the signal and to combine the attenuated previously received part of the signal and the noise signal to produce a combined signal the frame generator providing to the combined signal an increasing contribution from the noise signal relative to the previously received part of the signal as time passes.
According to another aspect of the invention there is provided a detector for detecting discontinuities in a signal comprising a sequence of frames and containing background noise in which the amplitude of the signal is measured to detect a sudden fall in amplitude and when an amplitude fall is detected its sharpness is determined and if the sharpness is sufficiently sharp a discontinuity indication is provided to control estimation of background noise.
According to another aspect of the invention there is provided a noise suppressor comprising an estimator to estimate background noise in a signal comprising a sequence of frames and containing background noise and a detector for detecting discontinuities in the signal in which the amplitude of the signal is measured to detect a sudden fall in amplitude and when an amplitude fall is detected its sharpness is determined and if the sharpness is sufficiently sharp a discontinuity indication is provided to control estimation of the background noise.
The invention is to detect artificial gaps in the signal which may have deliberately produced and but are not readily detectable because there is no discontinuity in the sequence of frames.
Preferably the discontinuity indication is used to control the rate at which an estimate of the background noise is up-dated. Preferably the rate is reduced when an amplitude fall is detected.
Preferably reduction of the rate at which the background noise estimate is up-dated is to protect the background noise estimate from being up-dated by something which is not noise being produced contemporaneously but may be based on noise from an earlier time. Preferably the background noise estimate is generated in a noise suppressor. Although the detector may be part of the noise suppressor, it may be a separate unit which simply gives and takes input to and from the noise suppressor. The decrease in amplitude may be due to one or more lost frames, or to an attenuation and repetition process used to mask such lost frame or frames or may be due to a reduction in real noise which is occurring contemporaneously which is contained in the signal. Alternatively, the detector detects a discontinuity caused by muting of the microphone. Reducing the rate of up-dating of the noise estimate results in the noise estimate being influenced less by part of the signal which is being dealt with at that particular time. In this way the noise estimate is still based on real background noise if it is still contained within the signal but its influence is reduced to deal with the possibility that real background noise is no longer contained within the signal at that time but some other signal, for example a repeated and attenuated frame is being used instead.
According to another aspect of the invention there is provided a method of detecting discontinuities in a signal comprising a sequence of frames and containing background noise comprising:
measuring the amplitude of the signal to detect a sudden fall in amplitude;
detecting when the amplitude falls;
determining the sharpness of the fall; and
if the sharpness is sufficiently sharp providing a discontinuity indication to control estimation of the background noise.
According to another aspect of the invention there is provided a mobile terminal comprising a noise suppressor in which the noise suppressor comprises an estimator to estimate background noise in a signal comprising a sequence of frames and a detector for detecting discontinuities in the signal the amplitude of the signal being measured to detect a sudden fall in amplitude and when an amplitude fall is detected its sharpness is determined and if the sharpness is sufficiently sharp a discontinuity indication is provided to control estimation of the background noise.
According to another aspect of the invention there is provided a communications system comprising a communications network having a noise suppressor and a plurality of communications terminals the communications system comprising an estimator to estimate background noise in a signal comprising a sequence of frames and a detector for detecting discontinuities in the signal in which the amplitude of the signal is measured to detect a sudden fall in amplitude and when an amplitude fall is detected its sharpness is determined and if the sharpness is sufficiently sharp a discontinuity indication is provided to control estimation of the background noise.
According to another aspect of the invention there is provided a noise suppression stage to act on a signal the noise suppression stage comprising a first windowing block to weight the signal by a first window function a transformer to transform the signal from the time domain into the frequency domain a transformer to transform the signal from the frequency domain into the time domain and a second windowing block to weight the signal by a second window function.
According to another aspect of the invention there is provided a two phase windowing method comprising the steps of:
weighting a signal in the time domain by a first window function to produce a frame;
transforming the frame into the frequency domain;
transforming the frame back into the time domain; and
weighting the frame by a second window function to suppress errors in matching between adjacent frames.
Preferably the method comprises the step of weighting by the windows after a speech encoding step. Alternatively, weighting may occur before a speech encoding step.
Preferably the window functions have a trapezoidal shape having a leading slope and a trailing slope. Preferably the first window function has a leading slope having a gradient which is shallower than that of the leading slope of the second window function. Preferably the first window function has a trailing slope having a gradient which is shallower than that of the trailing slope of the second window function. Having a relatively shallow slope in the first window function enables provides a good frequency transform. Having a relatively steep slope in the second window function provides good suppression of mismatch between adjacent frames in the time domain.
According to another aspect of the invention there is provided a mobile terminal comprising a noise suppression stage to act on a signal the noise suppression stage comprising a first windowing block to weight the signal by a first window function a transformer to transform the signal from the time domain into the frequency domain a transformer to transform the signal from the frequency domain into the time domain and a second windowing block to weight the signal by a second window function.
According to another aspect of the invention there is provided a communications system comprising a communications network having a noise suppression stage to act on a signal and a plurality of communications terminals the noise suppression stage comprising a first windowing block to weight the signal by a first window function a transformer to transform the signal from the time domain into the frequency domain a noise suppressor to suppress noise in the signal a transformer to transform the signal from the frequency domain into the time domain and a second windowing block to weight the signal by a second window function.
The signal may be noisy speech although speech may not be present all of the time.